Organic light emitting devices (OLEDs) represent a promising technology for display applications. A typical organic light emitting device includes a transparent first electrode, which usually acts as a hole-injecting anode; a luminescent region comprising one or more electroluminescent organic layer(s); and a second electrode (also called a xe2x80x9cback electrodexe2x80x9d), which usually acts as an electron-injecting cathode. In order to facilitate electron injection from the back electrode into the electroluminescent layer(s), the back electrode generally is formed of a low work function metal, and therefore is highly reflective. When a voltage is applied across the first and second electrodes, light is emitted from the electroluminescent layer(s) and through the transparent anode. When viewed under high ambient illumination, the reflective back electrode reflects a substantial amount of the ambient illumination to the observer, which results in higher ratios of reflected illumination as compared to the device""s own emission, which results in xe2x80x9cwashoutxe2x80x9d of the displayed image.
In order to improve the contrast of electroluminescent displays in general, light absorbing layers as described, for example, in U.S. Pat. No. 4,287,449, or optical interference members as described, for example, in U.S. Pat. No. 5,049,780, have been used to reduce the ambient illumination reflection. Both of these approaches involve the use of optical films made of materials that are generally non-conductive. These properties limit the applicability of such materials to organic light emitting devices, which, unlike inorganic electroluminescent phosphor devices, require the direct injection of charges from the electrodes to the electroluminescent layer(s). In addition, the fabrication of such optical films, essentially from dielectric inorganic materials, often requires the use of sputtering or electron beam evaporation techniques. These fabrication techniques require additional instrumentation to be incorporated into the otherwise relatively simple fabrication process of organic light emitting devices by thermal evaporation or spin coating techniques. Also, in cases using optical interference effects for achieving improved contrast, such as disclosed, for example, in U.S. Pat. No. 5,049,780, it is usually required to use more than one optical film in order to achieve the desired improvement, which is disadvantageous to the fabrication process. Furthermore, the resulting improvement in display contrast is inevitably dependent on the viewing angle.
Thus, there is a need, addressed by the present invention, for new OLEDs that avoid or minimize a number of the disadvantages described above for conventional electroluminescent devices.
Organic light emitting devices are disclosed in the following:
Pending U.S. Ser. No. 09/800,716 (titled xe2x80x9ccathodes for electroluminescent devices having improved contrast and reduced dark spot growthxe2x80x9d), assigned to Xerox Corporation; and
O. Renault et al., xe2x80x9cA low reflectivity multilayer cathode for organic light-emitting diodes,xe2x80x9d Thin Solid Films, Vol. 379, pp. 195-198 (Dec. 8, 2000);
International Application Publication No. WO 01/08240 A1; and
David Johnson et al., xe2x80x9cContrast Enhancement of OLED Displays,xe2x80x9d http://www.luxell.com/pdfs/OLED_tech_ppr.pdf, pp. 1-3 (April 2001).
The present invention is accomplished in embodiments by providing an organic light emitting device comprising:
a first electrode;
a second electrode; and
a luminescent region including an organic electroluminescent material between the first electrode and the second electrode, wherein one of the first electrode and the second electrode includes both a substantially transparent charge injecting layer adjacent to the luminescent region and an electrically conductive light absorbing layer.
There is also provided in embodiments an organic light emitting device comprising in sequence:
(a) a cathode including:
(i) an electrically conductive light absorbing layer, and
(ii) a substantially transparent electron injecting layer;
(b) a luminescent region including an organic electroluminescent material; and
(c) an anode that is substantially transparent to light.
There is provided in additional embodiments an organic light emitting device comprising in sequence:
(a) a cathode that is substantially transparent to light;
(b) a luminescent region including an organic electroluminescent material; and
(c) an anode including:
(i) a substantially transparent hole injecting layer, and
(ii) an electrically conductive light absorbing layer.